The present invention relates generally to metalization, and more particularly to a structure and method for preventing a phenomena called xe2x80x9ctungsten volcanoxe2x80x9d.
Metalization is an integrated circuit structure that connects individual devices by metal wires to form circuits. Pressure to decrease die area affects all circuits and circuit-related processes, including metalization. Specifically, in current chips, metal wires now carry electrical currents in the milliampere range, thereby resulting in extremely high current density. Therefore, defects in these metal wires can significantly impact the performance of the individual devices, the resulting circuits, and the chip itself. Thus, the goal of any metalization or associated process is to minimize both the number and size of such defects.
One of the most difficult problems in metalization is ensuring appropriate metal continuity at contact windows and vias. FIG. 1 illustrates a cross section 100 of a semiconductor device including two contact windows 101. To form the plugs within windows 101, three layers are typically provided: a titanium layer 102, a titanium-nitride (TiN) layer 103, and a tungsten layer 104. Titanium layer 102 acts as a glue layer between the underlying structures and TiN layer 103. As known to those skilled in the art, titanium-nitride is used as a nucleation layer to facilitate formation of tungsten layer 104. Tungsten, a refractory metal, provides a plug with low resistance to underlying layers and to subsequent metal layers, typically including aluminum (not shown). Tungsten layer 104 is typically formed through silane reduction of WF6.
Unfortunately, titanium reacts readily with WF6. Those skilled in the art have surmised that defects in TiN layer 103 allow WF6 to penetrate TiN layer 103 and to react with titanium layer 102 to form a defect known as xe2x80x9ctungsten volcanoxe2x80x9d. TiN layer 103, which is generally formed by sputter-deposition, has a high stress point at the edge of a xe2x80x9cstepxe2x80x9d 105 (i.e. the edge of a contact window). This stress may affect other areas of TiN layer 103, thereby resulting in numerous pin holes and cracks throughout TiN layer 103. TiN layer 103 tends to be porous at such steps, cracks, and pin holes and thus more vulnerable to WF6 penetration at these locations.
FIG. 2A illustrates the resulting peel back of TiN layer 103 after the fluorine reacts with titanium layer 102. During this reaction, the tungsten continues to be deposited irrespective of the peeling of TiN layer 103. Thus, tungsten forms on both sides of the peeling portion and eventually forms tungsten volcano 106, usually greater than 1 micron in size. For additional details regarding this phenomena, see ULSI Technology, by C. Y. Chang and S. M. Sze, pages 371-471, published by McGraw-Hill in 1996. A subsequent RIE etch cannot remove these volcanoes, thereby resulting in intra- or interlevel metal shorts.
A structure and method to prevent barrier failure is provided. The present invention replaces the standard titanium-nitride (TiN) barrier metal layer with two separately-formed TiN layers. The two TiN layers provide smaller and mismatched grain boundaries. During subsequent tungsten deposition using WF6, the WF6 finds it difficult to penetrate through these mismatched grain boundaries, thereby significantly minimizing the possibility of reacting with the underlying layer and forming a tungsten volcano. One embodiment includes a native or a grown oxide formed between the two TiN layers, thereby providing yet another diffusion barrier to the WF6 and acting as a glue layer between the two TiN layers. The present invention provides a thin and strong barrier metal layer with minimal barrier failures.